The convergence of spin-orbit configuration interaction calculations for TlH and (113)H

Abstract

To test the convergence of spin-orbit effects for molecules, the ground states of TlH and (113)H are calculated by configuration interaction(CI) calculations using relativistic effective core potentials with one-electron spin-orbit operators. The employed CI methods are the Kramers' restricted CI (KRCI) and the spin-orbit CI (SOCI) methods. The KRCI method includes the spin-orbit interactions in the generation of one-electron basis space through the use of the two-component molecular spinors obtained by the Kramers' restricted Hartree-Fock (KRHF) method, whereas the SOCI adds the spin-orbit term only at the CI level. For systems with heavy atoms, orbital relaxations due to the spin-orbit interaction could become sizable, resulting in slow convergences for the SOCI method. Spin-orbit effects on bond lengths and energies using single- and multireference CI calculations at the SOCI level of theory are evaluated and compared with KRCI results for TlH and (113)H. The spin-orbit effects on energies converge easily for TlH but slowly for (113)H. Especially, bond lengths do not converge for the seventh-row (113)H in our calculations. The present results imply that large-scale multireference SOCI calculations are necessary for some molecules to recover orbital relaxation effects due to large spin-orbit interactions in the SOCI scheme. In those cases, the KRCI scheme based upon two-component spinors will have advantages over SOCI and other one-component orbital based methods. (C) 2001 American Institute of Physics.